Summary

It is rarely possible to obtain reliable potassiumargon ages from sedimentary rocks. Glauconites generally give dates that are too low and very scattered, though the largest of a group of such measurements may be useful to define minimum ages of sediments that cannot be dated in any other way. Illite dates are subject to opposing systematic errors: because of this they may coincide roughly with the age of the sediment, but they cannot be regarded as reliable. Sylvines lose argon easily, though they may give good potassium-calcium dates.

From bentonites and volcanic ashes biotite, sanidine, and low-potassium feldspars have all been successfully dated. Contamination by old detritus is sometimes a problem, but the dates obtained from different minerals generally agree well, showing that the results are reliable. The wide geographical distribution of bentonites gives them a special value for time-scale work.

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